Mañuel
why not go 4.88's then? esp with 35's
other then that I got a fresh eng and I do get about 15mpg like you do jeffy but I do mostly town driving with the occasional highway mainly to go wheeling.
last time I actually did the math on the highway I got about 20-21mpg with all highway no town and it's because I did a long distance trip.. I was pulling a tiny trailer with a MiniZ on it "could not feel it" when on the highway cruising. but I must say I was doing mainly 50-55mph or less for speed. I was completely stock with 31x10.5r15's on stock gears.
as soon as I got to my destination I refueled and did the math for light to light with the little trailer stop and got dropped it to about 10mpg or less in town with the trailer on the back "can really feel the power loss and weight" in town.
so if you are bone stock you should be getting better then 8mpg.
engine wear like already said "compression check" etc should be checked first if you have not done that.
after that I would go after a clutch fan or something sticking on when driving highway thats not suppose to.
then I would go after such things like 02 sensors. I have even seen new ones be bad!!! scan tool could help this diag. possibly!!
no drive line bearing gone? high resistance in a diff or wheel bearing?
there is lots of stuff that can cause fuel consumption..